The program of any individual's vital cycle encoded in the genetic information is executed according to a time schedule that is the object of study of Chronogenetics. MZ twins provide the best evidence of the existence of this hereditary biological time and they represent, therefore, the natural test of Chronogenetics. Twin research can thus serve medicine in developing its most modern perspective, individual preventive medicine. Moreover, through approaches such as the Twins-Living-Apart Test, twin research can help to identify the influences of the environment on the health of man.

A new approach is proposed in twin research based on the study of twins who, though reared together, have subsequently lived apart for a period of at least five years. With respect to the more powerful study of twins reared apart, the twins living apart test has the advantages of being more realistic and affording easier access to sufficiently large samples. In this pilot study, the test has been applied to a sample of 92 monozygotic pairs now aged 35–45; 15 pairs were still living together and 77 had lived apart for over five years. As a first approach, comparisons have been made, in the cotwins of the two subsamples, with respect to the following traits: height, weight, presbyopia, presbyacusia, alcohol consumption, tobacco consumption, blood pressure (systolic and diastolic), dental caries, and hours of sleep.

A formal model is presented for the analysis of longitudinal twin data, based on the underlying analysis-of-variance model for repeated measures. The model is developed in terms of the expected values for the variance components representing twin concordance, and the derivation is provided for computing within-pair (intraclass) correlations, and for estimating the percent of variance explained by each component. The procedures are illustrated with physical growth data extending from birth to six years, and concordance estimates are obtained for average size and for the pattern of spurts and lags in growth. A test of significance is also described for comparing monozygotic twins with dizygotic twins. The procedures are particularly useful for assessing chronogenetic influences on development, especially whether the episodes of acceleration and lag occur in parallel for genetically matched twins. The model may be employed with psychological data also.